cache gstreamer config

Description

Instead of running xpra _sound_query every time we run the client or server, we could cache the results in a text file and just reload it.

We would need to ensure that the timestamp on this file is newer than the gstreamer plugins directory, so that we query it again whenever plugins are added or removed.
This is similar to what gstreamer does with its plugin registry (now found in .cache/gstreamer-1.0/) - can we re-use this mechanism without incurring the same startup cost?